1. Field of the Invention
This invention relates in general to metallurgical compositions useful as hardfacing materials, and in particular to improvements in composite hardfacing materials utilizing particles of tungsten carbide dispersed in a steel matrix.
2. Description of the Prior Art
Hardfacing compositions consisting of macroscopic tungsten carbide particles dispersed in a metal matrix have long been used to increase the wear resistance of manufactured articles. This general form of hardfacing material has been used in the earth boring industry on drill bits and on tool joint connectors of drill pipe.
There are two general types of tungsten carbide particles: (1) cast particles and (2) cemented or sintered particles.
Cast tungsten carbide particles are substantially pure tungsten carbide, having a hardness on the Knoop Scale of approximately 2700 and a melting temperature of approximately 500.degree. F.
Cemented or sintered tungsten carbide particles are a composition of microscopic primarily monotungsten, carbide and a binder metal selected from one or more of Group IV elements of the Periodic Table, namely iron, nickel and cobalt. The most commonly used binder is cobalt, which has a melting temperature and a resulting particle sintering temperature of about 2730.degree. F. The hardness of cemented tungsten carbide particles varies with the binder selected and with the percentage of binder used in relation to that of the tungsten carbide. Normally, the hardness of these particles ranges from 1400 to 1800 on the Knoop Scale.
In applying tungsten carbide particles to the matrix during the manufacturing process, a large number of macroscopic particles are only slightly embedded within, and thus protrude extensively from, the surface of the matrix. Such particles wear or fracture easily during drilling. To overcome this problem, a hand-held torch is sometimes used to wet the matrix at its outer surface to enable improved particle bonding with, and submergence within, the matrix. Hence the density of particles at the surface of the matrix increases, as does wear resistance. Also, bonding between the particles and matrix is improved. This is known as the "densification" technique, and proves satisfactory when using cast tungsten carbide particles, the melting temperature of which (5000.degree. F.) is sufficiently high to avoid damage by the heat generated with the torch.
Before the present invention, it was discovered that cemented tungsten carbide particles are surprisingly superior in some instances to cast tungsten carbide particles in a heat treated alloy steel matrix, even though they are softer and exhibit a lower tolerance for high temperatures. They are superior when used as ground gage surfaces of rolling cutter rock bits, for example. Unfortunately, cemented tungsten carbide particles cannot be utilized with the above described densification technique to increase the particle bonding and density in unground wear surfaces of tool joints, since the heat generated by the torch often exceeds the sintering temperature (2650.degree.-2700.degree. F.) of the particles. Exceeding this sintering temperature detrimentally alters the metallurgical composition of the cemented particles. As a consequence, the surprising improvement obtained with cemented tungsten carbide particles in an alloy steel matrix in drill bits has not been realized in tool joints because the tool joint hardfacing cannot be heat treated as it is in the case of the drill bit. Also the densification technique, used with cast carbide at the unground surface to gain additional wear resistance, cannot be used. Grinding of tool joint wear resistant surfaces to reach a layer of dense, well bonded particles is not economically feasible at this time.